Multiphase pipeline flows occur widely in the petroleum industry and in numerous chemical process plants. Of particular importance is the development of reliable models that can simulate their complex flow characteristics. A special challenge during the transport of gas condensate and oil well streams in the petroleum industry is the prediction of phase inventory and pressure loss in a long distance, large diameter multiphase pipeline at large water depths. Accurate flow models are essential to safe and cost efficient design and operation of field pipelines and topside downstream facilities.
Researches have been carried out for several decades in the past to understand the interactions of phases and flow characteristics. Different flow regimes were categorized to describe the interfacial macro- and meso-structures in two-phase gas-liquid and three-phase gas-liquid-liquid flows. Many experiments were conducted to study the flow details in each flow regime by using the state-of-art technologies, e.g. gamma densitometer, hot-film anemometer, LDA, PIV/PTV, ECT and X-ray computer tomograph. Information obtained has further strengthened our knowledge and provided insights to the local flow structures, such as turbulence, phase fraction and drop size. However, compared to the large amount of work in two-phase gas-liquid flows, few data are available for three-phase flows, and particularly for three-phase flows in large diameter pipes. Most of the few reported studies are only focused on the bulk parameters, e.g. pressure drop, mean holdups and flow regimes, based on the measurements with dual-energy gamma densitometer, quick closing valve and pressure transducers (see Sobocinski, 1955; Hall, 1992; Ackigoz et al., 1992; Pan, 1996; Wilkens, 1997; Morten et al., 2002; Odozi, 2000; Valle, 2000). Due to the lack of capable instruments, detailed information in a three-phase system on flow structure and phase distribution is even scarcer. Hu et al. (2005) reported the use of an X-ray tomograph in gas-oil-water flows in a 3 inch (76 mm) pipe, a system pioneered at imperial College London. Due to the lack of detectors with sufficient energy discrimination, the system resorts to moving filters to give alternating high and low energy exposures. Despite its successful application, the use of rotating filters led to a low time resolution, with 5 Hz as maximum sampling rate. The low temporal resolution has resulted in a loss of synchronization of structure between the horizontal and vertical measurements in fast flows, which brings difficulties in accurately reconstructing the cross-sectional tomographs.
Recently, an X-ray computer tomography system has been developed at Institute for Energy Technology which is based on the system pioneered at Imperial College but designed and manufactured with more up-to-date technologies, and which forms a basis for the present invention. The system is essentially a fast response two-phase system, giving a maximum sampling rate of 300 Hz. Latest applications of this system have shown versatile competence in measuring and visualizing complex multiphase flows (see Hu et al., 2009, 2010). Using such system, important flow behaviour of gas, oil and water phases in stratified and slug flows can be captured with a rather good accuracy.
The present invention improves the X-ray tomography measurement capability for gas-oil-water three-phase flows. In what follows, the next section illustrates the components and installation of the three-phase X-ray tomography system, followed by the description of the data analysis algorithm which may be used in the invention in the subsequent section. Then the next subsequent section shows the typical results that one can obtain using the present invention utilizing the described X-ray tomography system.
The Three-Phase X-Ray Tomography System
The X-ray tomography system comprises two or more generally identical channels, each channel comprising an X-ray source, an X-ray camera and computer controlled hardware, please see FIG. 1. The X-ray source has an anode-grounded, metal-ceramic vacuum tube with disc-shaped oil-filled insulator (model MB70-6-B-450, InnospeXion). A collimator is employed to provide a narrow X-ray beam. The two sources are arranged in fixed positions, one at the top and one horizontally relative to the enclosure, providing vertical and horizontal projections through the test pipe section, respectively, when the test pipe section is horizontally arranged. The X-ray detectors are high-resolution, high-sensitivity CdTe-CMOS linear arrays with an effective pixel area of 1500×56 pixels (150 mm×5.6 mm). The X-ray system is accompanied by a computer with a software suite which controls the data acquisition process, calibration of the detector arrays, data display, and storage of the results. The tomography system is fitted in a lead shielding box with steel sleeves on both sides for safe operation. This lead box and data collection PC's are built into a unit that is mounted on the IFE Well Flow Loop, and follows the loop as it is inclined from horizontal to vertical. In an embodiment the pipe test section is made of transparent PVC. The present embodiment of the X-ray system and its working principles are to some extent analogous to the one installed on the WASP flow rig at Imperial College London, UK, (see Hu et al., 2005). However, the present system does not use mechanical rotating sensor components, and thus can achieve the same maximum sampling frequency of 300 Hz for three-phase flows as for two-phase flows. This fast response allows for detailed studies of flows with rapidly changing structures, such as large wave and slug flows.
To achieve a three-phase measurement, one may use arrange a copper filter over a part of the camera (see FIG. 2). The copper filter absorbs (scatters) some of the X-rays, changing the energy spectrum seen by the camera. Since the lower energy (soft) X-rays are preferentially absorbed by the copper, the spectrum is said to be “hardened”. The X-ray absorption coefficients of the three working fluids oil, water and gas differ significantly in the low energy part of the spectrum. Oil and water have more similar coefficients in the high energy part of the spectrum. Thus, the high energy part of the spectrum serves to distinguish gas and liquid, while the difference between measurements in the high and low parts of the energy spectrum allows us to distinguish oil and water. Other sensor designs may discriminate between the energy levels of the incoming X-ray photons individually, and such sensors arrays may alternatively be employed.
During the design of an embodiment of the system, an optimisation process has been carried out to study the optimum filter thickness. Filters of different thicknesses in combination with the variance of the X-ray energy were tested and assessed for performance. It was found that for the current system, the optimum solution is the use of 0.1 mm thickness copper filter under the operation of the X-ray tube at 60 kV and 4 μA. FIG. 2a illustrates the schematic diagram for the use of a thin copper filter on the top of one section of Camera 1.
FIG. 2b shows a typical image produced by plotting a single frame of 150 mm length and 5.6 mm width (please notice the exaggerated width to length ratio) of the raw image visualized by Camera 1 as captured when the test section is filled with gas. To increase the total number of photons from hard beams that hit the detector, the filtered region is designed to cover ⅔ of the whole width (i.e. ˜3.7 mm). After removing the noisy pixels near the filter edge, an effective width of 1.6 and 2.9 mm is used for soft and hard beams, respectively.
With the experimental sensor setup wherein ⅔ of the width of the sensor cell array is filter covered and thus measures “hard” X-rays and the adjacent ⅓ width of the sensor cell array is not, and measures “soft (and hard)” X-rays, and the length of the sensor array (here 150 mm) is transverse to the general transport direction through the pipe section, it is evident that the data measured from “hard” and “soft” beams are spatially separated by a distance, averagely about 3 mm apart in the axial direction. This places a lower bound on the resolution of the system. For other sensor cell types than the presently used, such as for a sensor by Interon, wherein the same cell can discriminate X-ray energies of individual photons into separate bins, this is not a problem. In practice with the present setup using a copper filter, the signal represents a time-average over a short time interval (typically 3-10 ms). During this time, the fluids move a distance, where u is the local velocity of the fluid, which we assume is generally axial. For most of the flows we study, the velocity is in the range of >0.5 m/s, so that the spatial separation has a smaller influence on the results.
The present three-phase X-ray tomography system is installed on the high pressure Well Flow Loop at IFE, Norway. The rig comprises a 25 m long test section of 100 mm inner diameter that can be inclined at desired inclination angles, from 0 to ±90 degrees, please see details given in Hu et al., 2010. For higher inclination angles (>5 degree) a bend can be introduced between the first 10 meters of the test section and the rest, which can be inclined up to vertical. Light oil (Exxsol D80), tap water and high density gas (SF6) are used as test fluids. At the inlet to the test section the fluids enter with the phases separated by horizontal plates to reduce the influence of inlet geometry on downstream flows. At the outlet end of the test section the flow enters a pre-separator intended to generate an approximately constant liquid level and eliminate any suction effect when a slug passes into the downhill return pipe. The current X-ray tomograph system is located 2 m upstream of the pre-separator.